High accuracy beam placement for local area navigation

ABSTRACT

An improved method of high accuracy beam placement for local area navigation in the field of semiconductor chip manufacturing. Preferred embodiments of the present invention can also be used to rapidly navigate to one single bit cell in a memory array or similar structure, for example to characterize or correct a defect in that individual bit cell. High-resolution scanning is used to scan only a “strip” of cells on the one edge of the array (along either the X axis or the Y axis) to locate a row containing the desired cell followed by a similar high-speed scan along the located row (in the remaining direction) until the desired cell location is reached. This allows pattern-recognition tools to be used to automatically “count” the cells necessary to navigate to the desired cell, without the large expenditure of time required to image the entire array.

This application is a divisional application of U.S. patent applicationSer. No. 13/481,054 which claims priority from U.S. Pro. App.61/494,828, filed Jun. 8, 2011, and claims priority from and is acontinuation-in-part of U.S. patent application Ser. No. 13/285,229,filed Oct. 31, 2011, which is a continuation of U.S. patent applicationSer. No. 12/577,200, filed on Oct. 11, 2009, which claims priority fromU.S. Prov. App. No. 61/104,732, filed on Oct. 12, 2008, all of which arehereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to stage navigation and beam placement inparticle beam systems and, in particular, to high accuracy local areanavigation to a site of interest on a sample surface using acquisitionof a high-resolution image by FIB or SEM means.

BACKGROUND OF THE INVENTION

Semiconductor manufacturing, such as the fabrication of integratedcircuits, typically entails the use of photolithography. A semiconductorsubstrate on which circuits are being formed, usually a silicon wafer,is coated with a material, such as a photoresist, that changessolubility when exposed to radiation. A lithography tool, such as a maskor reticle, positioned between the radiation source and thesemiconductor substrate casts a shadow to control which areas of thesubstrate are exposed to the radiation. After the exposure, thephotoresist is removed from either the exposed or the unexposed areas,leaving a patterned layer of photoresist on the wafer that protectsparts of the wafer during a subsequent etching or diffusion process.

The photolithography process allows multiple integrated circuit devicesor electromechanical devices, often referred to as “chips,” to be formedon each wafer. The wafer is then cut up into individual dies, eachincluding a single integrated circuit device or electromechanicaldevice. Ultimately, these dies are subjected to additional operationsand packaged into individual integrated circuit chips orelectromechanical devices.

During the manufacturing process, variations in exposure and focusrequire that the patterns developed by lithographic processes becontinually monitored or measured to determine if the dimensions of thepatterns are within acceptable ranges. The importance of suchmonitoring, often referred to as process control, increases considerablyas pattern sizes become smaller, especially as minimum feature sizesapproach the limits of resolution available by the lithographic process.In order to achieve ever-higher device density, smaller and smallerfeature sizes are required. This may include the width and spacing ofinterconnecting lines, spacing and diameter of contact holes, and thesurface geometry such as corners and edges of various features.

As a result, careful monitoring of surface features is becomingincreasingly important. As design rules shrink, the margin for error inprocessing becomes smaller. Even small deviations from design dimensionsmay adversely affect the performance of a finished semiconductor device.

Accordingly, semiconductor customers are requiring high accuracy beamplacement to locate features such as single bit fails in memory arraysor locations for circuit edit. Beam shift navigation systems suffer fromsample drift, non-linearity in displacement, and are typically limitedin field of view. Typical sample stages used on particle beam systemsare only accurate to ±1-2 μm. Without a high accuracy stage (like alaser-encoded stage) it is not possible to drive the stage directly tothe location of interest with accuracy of 100 nm or less. Laser stagesmay have the capability for 100 nm accuracy but are expensive and limitthe system flexibility as the stage generally cannot be tilted, therebylosing functionality. Further, it is desirable to drive sample stageswithin an accuracy of approximately 30 nm, which is beyond thecapability even of typical laser stages.

In a typical memory array, it is often desirable to navigate to onesingle bit cell in the array, for example to characterize or correct adefect in that individual bit cell. A typical bit cell might be on theorder of 50 nm in size, while the total array might have an area of 100μm×100 μm. Navigation to an individual cell is currently done manually,by slowly moving the stage and counting the cells manually until thedesired location is reached. Such a manual process may take up to 10minutes to drive to a specific cell. Automatic navigation, for exampleusing pattern recognition to automatically count the cells, wouldrequire imaging the array at a resolution sufficient to resolve featuresdown to the cell size—in this example down to 50 nm. In order to havesufficient resolution to reliably perform pattern recognition on 50 nmcells, the array would preferably be imaged at a resolution of at least16K, possibly up to 64K or even higher. Such a high-resolution scan(64K) of a 100 μm×100 μm array (at a dwell time of 500 ns) would takeapproximately 34 minutes.

Thus, there is still a need for an improved method for high accuracynavigation to the site of interest within a local area on asemiconductor surface that will allows beam placement at an accuracybeyond the positional accuracy of the sample stage. There is also a needfor an improved method for high accuracy navigation that will allowrapid navigation to a single bit in an array without the necessity ofmanually counting. Further, there is a need for such improved methods tobe suitable for complete or partial automation.

SUMMARY OF THE INVENTION

It is an object of the invention, therefore, to provide an improvedmethod for high accuracy navigation to the site of interest within alocal area on a semiconductor surface, particularly in a particle beamsystem such as a FIB or SEM.

This invention demonstrates a method where high accuracy navigation tothe site of interest within a local area (e.g. 200 μm on a side) ispossible using imaging/patterning techniques provided there are visiblereference marks within the local area with a known coordinaterelationship to the target site. A high-resolution image of the targetarea of approximately 4096 pixels wide is first acquired. Two or morealignment features are located near the target area. The area ofinterest is overlaid with CAD polygons onto the image. Digital zoom isutilized to precisely locate the alignment points and a two or threepoint CAD polygon re-registration is performed.

Once the image and the coordinate system have been properly aligned, thealignment can be transferred to the sample itself by way of one or moretransfer fiducials. One or more readily identifiable features on thesample in the vicinity of the feature of interest are selected and theoffset between the transfer fiducial(s) and the feature of interest isrecorded. The sample can then be re-imaged at a much smaller field ofview based upon the coordinate system alignment. Once the transferfiducials are identified in the second image, the recorded offsets canbe used to locate the feature of interest and accurately position theparticle beam.

According to preferred embodiments of the present invention, thecombination of large area, high resolution scanning, digital zoom, andregistration of the image to an idealized coordinate system enablesnavigation around a local area without relying on stage movements. Oncethe image is acquired any sample or beam drift will not affect thealignment.

Preferred embodiments of the present invention can also be used torapidly navigate to one single bit cell in a memory array or similarstructure, for example to characterize or correct a defect in thatindividual bit cell. High-resolution scanning is used, not to scan theentire array, but instead to scan only a “strip” of cells on the oneedge of the array (along either the X axis and the Y axis) to locate arow containing the desired cell followed by a similar high-speed scanalong the located row (in the remaining direction) until the desiredcell location is reached. This allows pattern-recognition tools to beused to automatically “count” the cells necessary to navigate to thedesired cell, without the large expenditure of time required to imagethe entire array. Using preferred embodiments of the present invention,a single bit cell can typically be located automatically in less than 5minutes, as compared to more than 30 minutes for some prior art methods.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a flowchart showing the steps of high accuracy beam placementfor local area navigation utilizing stageless navigation according to apreferred embodiment of the present invention.

FIG. 2 shows an image of a sample including a target and alignmentfeatures to be used in the initial image/CAD overlay registrationaccording to a preferred embodiment of the present invention.

FIG. 3 shows the image of FIG. 2 with an overlay showing the CADpolygons prepared from the CAD data superimposed on the image.

FIG. 4 shows the image and the CAD overlay of FIG. 3 at a highermagnification using digital zoom.

FIG. 5 shows the image of FIG. 4 in which a first point in the CADoverlay and the corresponding feature in the image have been selectedfor a registration of the image and the CAD overlay.

FIG. 6 shows the image of FIG. 4 in which a second point in the CADoverlay and the corresponding feature in the image have been selectedfor a registration of the image and the CAD overlay.

FIG. 7 shows the image of FIG. 4 in which a third point in the CADoverlay and the corresponding feature in the image have been selectedfor a registration of the image and the CAD overlay.

FIG. 8 shows the image and CAD overlay after 3-point registration hasbeen completed.

FIG. 9 shows a second charged particle beam image according to thepresent invention where the image has a smaller field of view andincludes the location of the feature of interest and at least onefeature that can be used as a transfer fiducial.

FIG. 10 shows a charged particle beam image according to the presentinvention where a fiducial frame has been milled around the location ofthe feature of interest.

FIG. 11 is a flowchart showing the steps of high accuracy beam placementfor local area navigation according to a preferred embodiment of thepresent invention.

FIG. 12 shows a typical dual beam FIB/SEM system that could be used toimplement aspects of the present invention.

FIG. 13 is a flowchart showing the steps of navigating to a single bitcell in a memory array or similar structure according to a preferredembodiment of the present invention.

FIG. 14 shows a schematic representation of a memory array containingindividual cells.

FIG. 15 is a close-up view of a portion of the memory array of FIG. 14.

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are directed at methodsfor high accuracy beam placement for local area navigation in the fieldof semiconductor chip manufacturing. This invention demonstrates amethod where high accuracy navigation to the site of interest within arelatively large local area (e.g. an area 200 μm×200 μm) is possibleeven where the stage/navigation system is not normally capable of suchhigh accuracy navigation.

According to preferred embodiments of the present invention, ahigh-resolution image of a relatively large target area (a larger areaincluding the location of a feature of interest and one or more suitablealignment marks) is first acquired. For example, a suitablyhigh-resolution area might be 250 μm wide with a resolutionapproximately 4096 pixels wide. According to one preferred embodiment,the area of interest is overlaid with CAD polygons and a two or threepoint CAD polygon re-registration is performed. Digital zoom is thenutilized to precisely locate the area containing the feature ofinterest. An additional CAD registration process can be performed forgreater accuracy. One or more suitable transfer fiducials are thenlocated or created near the feature of interest and the offset betweenthe fiducial and the feature of interest in the large field of viewimage is recorded. A smaller field of view image is then acquired thatis suitable for performing the inspection/investigation. The transferfiducial is identified in this image and the offset used to accuratelylocate the feature of interest.

The combination of large area, high-resolution scanning, digital zoom,and registration of the image to an idealized coordinate system enablesnavigation around a local area without relying on stage movements. Oncethe image is acquired any sample or beam drift will not affect thealignment. Preferred embodiments thus allow accurate navigation to asite on a sample with sub-100 nm accuracy—with some preferredembodiments allowing navigation within 30 nm—even without ahigh-accuracy stage/navigation system. In other words, according topreferred embodiments of the present invention, the sample stage canhave a positioning accuracy or error of ±500 nm or greater while thefeature of interest can be located and the particle beam can bepositioned relative to the sample with a positioning accuracy of ±100 nmor better (i.e., within 100 nm or less). More preferably the feature ofinterest can be located (i.e., the particle beam system can navigate tothe location of the feature of interest on the sample) with an accuracywithin ±30 nm or better. Even where the sample stage has a positioningaccuracy or error of ±100 nm or greater, preferred embodiments of thepresent invention allow the feature of interest to be located and theparticle beam to be positioned relative to the sample with a positioningaccuracy of ±300 nm or better.

A preferred method or apparatus of the present invention has many novelaspects, and because the invention can be embodied in different methodsor apparatuses for different purposes, not every aspect need be presentin every embodiment. Moreover, many of the aspects of the describedembodiments may be separately patentable.

FIG. 1 shows a flowchart showing the steps of high accuracy beamplacement for local area navigation according to a preferred embodimentof the present invention. In step 10, the sample is loaded into atypical prior-art charged particle beam system (such as the FIB/SEMillustrated in FIG. 12 and described below) by mounting the sample onthe system stage. The charged particle beam system according to apreferred embodiment of the present invention can be a focused ion beamsystem, an electron beam system, or a dual beam FIB/SEM system. Thesample can be loaded manually or automatically, for example by anautomatic handler system.

Preferred embodiments of the present invention do not require the use ofa high-accuracy stage such as a laser stage. When a sample or workpieceis loaded into a charged particle beam system for analysis orprocessing, it can be very challenging to drive the stage to the preciselocation of a feature of interest. Typical sample stages have anaccuracy of approximately 1-2 μm. In other words, when such a typicalstage is moved to a particular coordinate, the error in position can beup to ±1-2 μm. (When expressed in this fashion, a larger number means aless accurate stage.) Advanced high-accuracy stages, such as laserinterferometer stages (hereinafter “laser stages”) which are capable ofnavigation within accuracy of 100 nm or less are very expensive. Laserstages also have some significant disadvantages in that they typicallydo not tilt and they are not available on the majority of chargedparticle beam systems currently in use. The present invention provides amethod of navigation with sub-100 nm accuracy that does not require ahigh-accuracy laser stage. Preferably, embodiments of the presentinvention provide a method of sub-100 nm navigation or beam placementusing a sample stage with an accuracy (positional error) of ±500 nm.More preferably, embodiments of the present invention provide a methodof sub-100 nm navigation or beam placement using a sample stage with anaccuracy (positional error) of ±1-2 μm or greater.

After the sample is loaded, in step 12, the sample is aligned usingknown methods, for example by a typical three-point lock on the cornersof the die. This alignment can also be accomplished manually, forexample by an operator using an optical microscope, or automatically,for example by using an automatic handler robot which locates a notch orflat edge of the sample in order to determine the proper orientation.

In step 14, the stage is positioned so that the location of the featureof interest is within the target area to be scanned by the chargedparticle beam (the field of view). (In some cases, the feature ofinterest may not actually be visible in the image, such as for examplewhen the feature of interest is buried.) This positioning can beaccomplished, for example, by storing and using positional coordinatesor computer-aided design (CAD) data. The field of view should be largeenough so that, considering the accuracy of the stage/system being used,it is assured that the feature of interest is within area to be imaged,preferably along with one or more appropriate alignment featuressuitable for use in registering the image with a coordinate systemrepresenting the locations of features on the sample surface (asdiscussed in greater detail below). More preferably, the field of viewwill include at least three features suitable for use as alignmentfeatures. Suitable features should be easily recognizable in the sampleimage and in the coordinate system overlay.

In step 16, sample is imaged at high resolution with the chargedparticle beam. The image must be of sufficient (high enough) resolutionso that the pixel size is comparable to the placement precisionrequired. The image resolution is preferably high enough that the pixelsize allows the alignment marks to be identified and their locationsaccurately determined. More preferably the resolution is high enoughthat the pixel size is the same size of smaller than the size of thealignment features. For example, in a preferred embodiment of thepresent invention, this means that for a 250 μm wide image a resolutionof 4096 (or greater) pixels could be used, resulting in pixels of about50-60 nm in size. As a result, alignment features larger than 50-60 nmcould easily be identified. Other preferred embodiments of the presentinvention use image resolution resulting in a pixel size of 10-100 nm,more preferably pixel sizes of 30-60 nm.

In some preferred embodiments, resolution resulting in a pixel size thatis larger than the size of the feature of interest could also be used,although the larger pixel size would contribute to positional error. Forexample, if the pixel sizes in the embodiment described in the precedingparagraph were used (50-60 nm) with an alignment feature 30 nm in size,there would be no way to determine where the alignment feature waspositioned within the pixel. As a result, the positional error(resulting solely from the pixel size) of the alignment feature could beas much as 20-30 nm (50-60 nm minus 30 nm). Since this degree ofaccuracy is still more than can be achieved even by typical laserstages, this degree of accuracy will be acceptable in many cases.

The location of the target (feature of interest) and preferably thelocation of the alignment features should also be known in some form ofcoordinate system. In preferred embodiments, the locations can bedetermined from a CAD overlay (as described in greater detail below) orx, y coordinates or else the structure is a repeating array.

The image should be of sufficient resolution that the pixel size is atleast comparable to the precision required. For example, in onepreferred embodiment, a 250 μm wide image would be approximately 4096pixels wide, resulting in pixels about 60 nanometers in size. This wouldbe suitable for imaging or processing features, such as the alignmentpoints, that are 60 nanometers in size or larger. However, a higherresolution (and resulting smaller pixel size) would be required forfeatures that are smaller than 60 nm.

There is a direct balance between field of view (also called horizontalfield width or HFW) image resolution and pixel spacing: HFW=(pixelspacing)*(number of pixels). To navigate over an area much larger than250 μm×250 μm ones demonstrated here would result in a likely reductionin the accuracy obtainable due to larger pixel spacing and possible scandistortions unless the resolution of the patterning engine was increasedto 8 k or 16 k wide images. In some cases, mapping may be required tounderstand any scan distortions/non-linearities.

FIG. 2 shows an image of a sample obtained as described above by drivingthe stage to the coordinates for a feature of interest and threealignment features to be used in the initial image/CAD overlayregistration (described below). As shown in FIG. 2, the target 201(containing the feature of interest) and the three alignment features(202, 203, 204) are all via fields, which have been partially exposed ona semiconductor surface by milling through a surface layer. The field ofview should be large enough so that, considering the accuracy of thestage/system being used, it is assured that the location of the featureof interest is within the area to be imaged. For example, in a particlebeam system with a sample stage having an accuracy of ±2 μm, the fieldof view for a feature of interest and three alignment featuresapproximately 2000 nm apart should be at least 8 μm×8 μm to ensure thatthe feature of interest is within the field of view imaged. Typically,however, a much larger field of view of approximately 125 μm×125 μmcould be used according to the present invention.

Referring again to FIG. 1, in step 17 an overlay showing the CADpolygons 330 (idealized geometric shapes that represent the locations offeatures on the sample or workpiece) can be constructed from thecomputer-aided design (CAD) data for the elements/features on thesemiconductor sample and superposed on the charged particle beam imageof the sample. Such a coordinate system overlay is shown in FIG. 3. Ifnecessary, an initial registration between the CAD overlay and the imagecan be performed as described below. Note that in FIG. 3, a number ofthe semiconductor features indicated by the CAD polygons have not beenexposed and are still buried under the surface layer.

Typically, as shown in FIGS. 3 and 4, the initial positioning of theoverlay relative to the image will likely be somewhat inaccurate. Inoptional step 18, digital magnification can be used to zoom in on thetarget and the alignment points. Once the target area has been scanned,the use of digital zoom allows a user to “navigate” the scanned image tolocate fiducials or the feature of interest. It is much quicker andeasier to navigate around in the image than it would be to navigate bymoving the stage and re-imaging the sample. Digital zoom (for example,on the order of 8:1 zoom in FIG. 4) allows the user to quickly locatethe general area containing a feature of interest and then to zoom in tomore accurately locate the feature of interest, alignment features foruse with the coordinate overlay, and/or transfer fiducials as describedbelow. In FIG. 4, the magnified image makes it clear that the vias 340are not properly aligned with the CAD polygons 330.

In preferred embodiments of the present invention, the use of digitalzoom allows an operator to zoom in on the image (and the CAD overlay) ator near the feature of interest in order to perform a coordinate systemregistration between the image and the overlay in order to moreaccurately align the image with the CAD overlay. As persons of ordinaryskill in the art will recognize, the use of digital zoom helps overcomeinherent limitations in the level of detail that can be visuallydisplayed to a human operator, for example on a computer monitor. Insome preferred embodiments of the present invention, however, automatedcomputer control can be used instead of human operators, for example byusing methods of computer analysis of image data such as imagerecognition/machine vision. The use of digital zoom would obviously notbe necessary for automated embodiments.

The alignment points and corresponding elements on the coordinate systemoverlay can then be identified, for example by using a computer pointingdevice such as a mouse and on-screen cursor. FIG. 5 shows the first stepin the re-registration process. Using digital magnification, the usercan zoom-in on the image and CAD overlay and specify a local offset forvarious regions within the image. The region that is being digitallymagnified is shown as a square 550 in thumbnail view 551. As shown inFIG. 5, the image has been digitally zoomed at alignment point 202.Referring again to FIG. 1, in step 20—as illustrated in FIG. 5—anoperator can click (with a mouse or other suitable pointing device) thecenter 544 of one of the CAD polygons 330 and then click in the center542 of the corresponding feature (via 340) in the sample image. Theprocess is can then be repeated for alignment feature 204 as shown inFIG. 6 (step 22) and at alignment feature 203 as shown in FIG. 7 (step24).

Once the locations of the alignment features and the correspondingelements in the coordinate system overlay have been identified, theoffset or overlay error in the target region between the alignmentpoints on the image and the CAD overlay can then be measured in step 26.The error in FIG. 4 is about 1.4 μm in the target region. To correctthis error, a three-point re-registration will be performed in step 28and the CAD overlay is stretched, rotated, and/or shifted to create amatch to the image.

As persons of ordinary skill in the art will recognize, the offseterrors between the CAD overlay and the image can arise from manysources: operator error in the original stage lock, imprecisecalibration of the FIB image (magnification and/or rotation), local diedistortions, or non-linearities in the ion column deflection system, toname a few. Whatever the source of the errors, it is usually impossibleto achieve perfect correspondence between a CAD overlay and every pointwithin a large field-of-view FIB image. One solution to this problem isto perform a 3-point re-registration that stretches, shifts, and/orrotates the CAD overlay as needed to create a customized match of aparticular FIB image. This type of image registration is discussed indetail in U.S. Pat. No. 5,541,411 to Lindquist et al. for“Image-to-Image Registration Focused Ion Beam System,” assigned to FEICompany of Hillsboro, Oreg., also the assignee of the present invention,and incorporated herein by reference.

As described by Lindquist and as shown by FIGS. 5-8, the registrationoperation comprises selecting, for example, three points on the particlebeam image and three corresponding points on the other image to bealigned (such as a CAD overlay according to a preferred embodiment ofthe present invention). The alignment points can be selectedinteractively via use of mouse in conjunction with visual feedback fromdisplay. For example, if alignment points R1, R2, and R3 are chosen asalignment points visible in the particle beam image, the threecorresponding points (C1, C2, C3) are selected the coordinate systemoverlay for which registration is desired. Once the corresponding pointsin the image and coordinate system are chosen, the process determines atransform T between the points of the reference image (R1, R2, R3) andthe points on the prior image (C1, C2, C3) such that T(C1)=R1, T(C2)=R2,and T(C3)=R3. Since the coordinate locations of the points are knownwithin the particular coordinate system of the screen, thetransformation between points is readily determined by linear algebramethods.

Once the transform operation T has been determined, then a new empty(i.e., blank) image is created, then a loop is entered and a first pixelis selected in the new image. Once the pixel is selected, adetermination is made as to whether all pixels have been processed. Ifall pixels have been processed, then the registration is completed andthe process exits. However, if all pixels have not been processed, thenthe process continues using the coordinate transform T and acorresponding pixel from the prior image is selected. The selected pixeldata from the prior image is then read from the prior image into theselected pixel position of the new image. If the transform has mappedthe selected position of the new image outside of the boundary of theprior image, then null data is placed in the new image position. Thisnull data could comprise a blank representation or a black backgroundrepresentation, for example. Next, the process loops back, to againselect a next pixel in the new image and the process continues in aniterative manner until such time as all pixels in the new image havebeen processed.

In a preferred embodiment the calculation of offset and the registrationof the image and CAD polygons are performed by way of an automatedcomputer script. After registration is complete, the overlay and targetvias are now properly aligned, as shown in FIG. 8.

The new image may be somewhat skewed relative to the original images,since the transform operation will accommodate translation, rotation,scaling, and tilt angle differences between the images. According to theabove-described steps, image-to-image registration is preferablyaccomplished by selecting three corresponding alignment points in thecharged particle image and the coordinate system, although differentnumbers of alignment points could be used with more points providing amore accurate alignment. A transformation between the correspondingalignment points is determined and applied to the images to beregistered to produce an appropriately registered output image.Alternatively, an optional step to improve accuracy is to imageadditional sites with other alignment features close to the target site.The target site is then determined by averaging the position indicatedby the separate images. This step is especially desirable to providinggood accuracy when a single alignment features is used per image. Thesteps are suitably performed by a computer processor, wherein thevarious images are bit-mapped images stored in image buffer and shown onan appropriate display.

Once the image and the coordinates system have been properly aligned,that alignment then needs to be “transferred” to the sample itself.According to preferred embodiments of the present invention, this can beaccomplished by the use of transfer fiducials. Factors such as systemdrift and image shifts and scaling differences when changing fields ofview make the positioning of a charged particle beam somewhat variableat the nanometer scale. The use of transfer fiducials allows independentreference points to quickly and precisely locate a feature of interest.A transfer fiducial can either be an existing and easily identifiablefeature on the sample (as seen in the image) or one created by theFIB/SEM as part of the alignment process. An existing feature suitablefor use as a transfer fiducial should be a unique feature within thefield of view that can be consistently identified. A preferred fiducialwill also allow the beam location to be pinpointed in both the x and ydirections. For example, one suitable fiducial might be the intersectionof two lines (a cross-shaped fiducial). A suitable fiducial could alsobe an irregularity in the sample or even a piece of dirt or debrislocated within the field of view.

Referring again to FIG. 1, in step 30, after navigating to the locationof the feature of interest within the image, one or more suitabletransfer fiducials are identified in the image, and the offset betweenthe fiducial(s) and the feature of interest within the image isrecorded. Again, digital zoom can be used to magnify the image of thearea at the location of the feature of interest to aid in the locationof appropriate transfer fiducials. Preferably, two or more transferfiducials will be used. In general, the greater the number of transferfiducials used, the greater the accuracy of the location of the featureof interest.

If an appropriate structure is not present on the surface of the sample,in step 32 a fiducial mark can be created at a location within the fieldof view but separated from the feature of interest, preferably in alocation that will not damage the point of interest. For example, afiducial marker can be created on the sample by FIB milling or FIB/SEMdeposition close to the target site. A fiducial may be created using anysuitable method, including for example, focused ion beam sputtering,surface staining with an ion beam, gas-assisted etching or deposition,or electron beam induced gas-assisted etching or deposition. In manycases, fiducial formation by deposition will be preferable because it isless invasive (causes less damage to the sample surface) and provides abetter contrast (because a different material is used). The fiducial canbe made of a shape that is readily distinguishable so that it can beconsistently identified and located.

Where an appropriate fiducial is created, in step 33, the sample surfaceshould be re-imaged after the fiducial is created. The alignment steps16-28 described above can then be repeated with the new image.

In step 34, once suitable transfer fiducials are identified, the offsetbetween the transfer fiducials and feature of interest is determined andrecorded (preferably in computer memory). FIG. 9 shows a sample wherethe corner 102 of a cross pattern has been located using the methoddescribed above and two milled fiducials 104 and 106. Using the methodsdescribed herein, the location of corner 102 was experimentallydetermined within <100 nm.

Again, the greater number of fiducials created and used, the greater theaccuracy of the beam placement in relation to the feature of interest.In a preferred embodiment, a frame or box can be created, for example byion milling, completely around the location of the feature of interest(although as shown in FIG. 9, frame fiducial 104 was milled aroundalignment mark 106 rather than the feature of interest 102). Such amilled fiducial frame is shown in FIG. 10, where a buried feature ofinterest (not yet visible because of the surface layer) is locatedwithin fiducial frame 404. Alignment features 402, 403, 404 (which havealready been exposed) are also visible in FIG. 10. By using a framingfiducial as shown in FIG. 10, once the feature of interest within theframe is exposed (preferably by milling/etching away the surface layerwithin the frame) the feature offset can be determined for any pointalong the frame, essentially providing an infinite number of fiduciallocations from which the offset can be determined. This provides for amuch more accurate determination of the offset between the fiducial andthe feature of interest.

Referring again to FIG. 1, once the appropriate fiducials have beenselected or created, in step 36, the sample is re-imaged at the locationof the feature of interest. Preferably, a significantly smaller field ofview is used for this imaging step. Typically, the field of view will beone that is typical for the desired processing of the feature ofinterest, for example a 10 μm×10 μm field of view. This smaller field ofview can be used with greater confidence because, after the registrationsteps described above, the location of the feature is known with enoughaccuracy to ensure that the feature will be within the smaller field ofview. The transfer fiducial(s) can then be readily identified in the newimage in step 38. And in step 40, the recorded offset(s) are used toeasily and accurately locate the feature of interest. Once the beam iscorrectly positioned, in step 42, the particle beam can be used toprocess the sample by, for example, milling the sample, depositingmaterial onto the sample, or imaging and performing metrology on thesample.

FIG. 11 is a flowchart showing the steps of creating one or more samplesaccording to another preferred embodiment of the present invention(without using a visual overlay of CAD polygons). Preferably, theprocess described in FIG. 11 can be completely or partially automated.

In step 110, a sample is loaded into a particle beam system. Forexample, a suitable sample could be a semiconductor wafer, which couldbe loaded into a dual beam FIB/SEM having a sample stage with anaccuracy of only 1-2 μm. The known coordinates of a feature of interestare then used to position the stage so that the feature of interest iswithin the field of view of the particle beam. Because of the lowaccuracy of a typical sample stage, the precise location of the featureof interest cannot be identified with sufficient accuracy usingcoordinates alone.

In step 111, a high-resolution image (e.g. 4096 pixel wide) image of thetarget area is acquired, including two or more alignment points (e.g.identifiable features such as the corners of a 200×200 μm square area).In step 112, the suitable alignment points are located. As describedabove, suitable alignment features can be identified in the sample imageand corresponding elements identified in coordinate system dataspecifying the locations of features on the sample (such as CAD data forthe particular semiconductor wafer).

According to a preferred embodiment of the present invention, suitablealignment features also can be selected automatically using imagerecognition software. Suitable image recognition software is available,for example, from Cognex Corporation of Natick, Mass. Image recognitionsoftware can be “trained” to locate the suitable alignment features byusing sample images of similar features or by using geometricinformation from CAD data. This can be especially desirable where anumber of similar samples are to be processed (for example a largenumber of semiconductor wafers having the same design). Automated FIB orSEM metrology can also be used to identify or help identify thealignment features. Metrology may consist of image-based patternrecognition, edge finding, ADR, center-of-mass calculations, blobs, etc.Suitable software to implement fully or partially automated imageprocessing, metrology, and machine control according to the presentinvention preferably provides pattern recognition and edge detectiontools, along with “do while” looping capabilities, such as the IC3D™software also available from FEI Company, the assignee of the presentinvention.

In step 114, the image and the coordinate system of the alignment pointsare aligned based on calculation of the offset as described in greaterdetail above. In step 116, this alignment is used to calculate thelocation of desired feature of interest in image. In optional step 118,a re-registration can be performed in the vicinity of the feature ofinterest.

In step 120, it is determined whether suitable transfer fiducials arepresent on the sample surface. Again, transfer fiducials can be selectedautomatically using image recognition software. Alternatively, suitabletransfer fiducials could be selected initially by an operator and theimage recognition software “trained” to locate the suitable transferfiducials in subsequent samples.

If suitable transfer fiducials not present, in step 122, a physicalfiducial is created to allow target location. The fiducial can becreated to the side of the target area, for example with FIB, SEM, orother known methods, as described above. The offset to the marker shouldbe large enough to make sure that the target site will not be damaged orobscured by the marker. Depending upon the accuracy of the stage, thefiducial might need to be formed several μm away from the feature ofinterest. The locations for created transfer fiducials could bespecified by an operator, for example, by using a mouse to drag avirtual box around the desired fiducial location. Automated metrologysoftware could then precisely measure the location of the fiducial withrespect to identifiable features at the sample location (for example 15nm from the right edge of a particular feature). For processingsubsequent samples, a fiducial could then be automatically created atthe precise location specified. A fiducial location could also bespecified using CAD data to specify the location of the fiducial withrespect to a particular structure on the wafer surface. As long as thetransfer fiducials were created far enough away from the feature ofinterest (considering the accuracy of the stage navigation) suitabletransfer fiducials could be safely created by this type of automatedprocess.

Where transfer fiducials are created, a second high-resolution image canbe acquired and process steps 111-118 repeated in order to properlyregister the second image (showing the transfer fiducials) with thecoordinate system.

In step 124, the suitable transfer fiducials (whether pre-existing orcreated) are then identified and the offset(s) between the transferfiducials and the feature of interest are recorded.

In step 126, the recorded fiducial offsets are used to accurately locatethe feature of interest so that the particle beam can be preciselypositioned. One method of performing the final alignment would be tocreate an overlay over the fiducial in the high-resolution image, forexample by drawing a pattern box. If the user now acquires an image athigher at higher magnification, the target site can be found by liningup the marker and the pattern, e.g. by using beam shift. By using thisnovel process, the feature of interest can be located and particle beamplacement controlled within ±30 nm or even less. This allows the sampleto be processed, in step 128, with very accurate beam placement eventhough the sample stage alone is not capable of such precise navigation.In step 130, it is determined whether there are other samples to beprocessed. If yes, subsequent samples are loaded into the particle beamsystem and steps 111-130 are repeated (preferably automatically asdescribed above). If not, the process stops.

Preferred embodiments of the present invention can also be used torapidly navigate to one single bit cell in a memory array or similarstructure, for example to characterize or correct a defect in thatindividual bit cell. A typical bit cell might be on the order of 50 nmin size, while the total array might have an area of 100 μm×100 μm. Insystems without an expensive, high-accuracy laser stage, navigation toan individual cell location is currently done manually, by slowly movingthe stage and counting the cells manually, along both the X-axis andY-axis, until the desired location is reached. Such a manual process maytake up to 10 minutes to drive to a specific cell, and is also prone toerror resulting from miscounting or accidental misalignment. Smoothjogging stages can minimize such counting errors, but these types ofstages are expensive and not in widespread use.

Automatic navigation, for example analyzing the image data and/or usingpattern recognition to automatically count the cells, would requireimaging the array at a resolution sufficient to resolve features down tothe size of the minimum repeating dimension of the cells—in this exampledown to 50 nm. In order to have sufficient resolution to reliablyperform pattern recognition on 50 nm cells, the array would preferablybe imaged at a resolution of at least 16K, possibly even as high as 64K.Such a high-resolution scan (64K) of a 100 μm×100 μm array (at a dwelltime of 500 ns) would take approximately 34 minutes.

Preferred embodiments of the present invention, however, usehigh-resolution scanning, not of the entire array, but only of a “strip”of cells on the edges of the array (along either the X axis and the Yaxis) to locate a row containing the desired cell followed by a similarhigh-speed scan of a strip of cells along the located row (in the otherdirection) until the desired cell location is reached. Preferably, the“strips” are substantially smaller than the size of the array. Forexample, a preferred strip would be less than 10 cells wide, morepreferably less than 5 cells wide. For a typical array, this would makethe strips less than 10% of the size of the array. This allowspattern-recognition tools to be used to automatically “count” the cellsnecessary to navigate to the desired cell, without the large expenditureof time required to image the entire array. Using preferred embodimentsof the present invention, a single bit cell can typically be locatedautomatically in less than 5 minutes.

FIG. 13 is a flowchart showing the steps of navigating to a single bitcell in a memory array or similar structure according to a preferredembodiment of the present invention. Preferably, the process describedin FIG. 13 can be completely or partially automated.

In step 150, a sample is loaded into a dual beam SEM/FIB particle beamsystem, and the beam system is directed at the region of interest. Forexample, a suitable sample could be a semiconductor wafer having amemory array or similar structure as a region of interest. In step 152,the SEM is used to image the sample to find and identify the desiredarea (i.e., the target area) containing the desired cell. For example,the SEM could be used to image a 200 μm×200 μm area using a normalresolution of 1K×1K. It would not be possible to identify individual 50nm cells in such an image because the pixel size would be 200 nm. FIG.14 shows a schematic representation of such an area 1400, in which theindividual “squares” represent individual cells in a memory array orsimilar structure. FIG. 15 shows a close-up view of the area 1400 withinthe dashed box 1408.

In step 154, the XY cell count for the desired cell is provided. Forexample, a typical cell count number within the field of view might becell x=2478, y=399, and this count could be provided by an operator orby CAD data. In step 156, the corner of the array (lower left, lowerright, upper left or upper right) closest to the desired cell isdetermined, for example by automatic software. Next, in step 157, one ormore fiducials can be formed on the substrate, one preferably in thevicinity of the first “row” to be counted (for example, at or near rowy=399) and one near the location of the desired cell. Skilled personswill recognize that the first “row” to be counted can be either the Xvalue or the Y value, with the other value being used to count along thelocated row to reach the requested cell location. Because the preciselocation of the requested cell has not yet been determined, thepositioning of the fiducial(s) can be somewhat rough as long as both thefiducial and desired row/cell location will definitely be within thefield of view.

Referring also to FIG. 15, fiducial markers 1410, 1412 will preferablybe placed so that they are in the image “strip,” discussed below, butseparated from the desired cell, preferably in a location that will notdamage the feature of interest. Depending upon the accuracy of thestage, the fiducial might need to be formed several μm away from thefeature of interest to make sure the feature is not inadvertentlydestroyed. A fiducial may be created using any suitable method,including for example, focused ion beam sputtering, surface stainingwith an ion beam, gas-assisted etching or deposition, or electron beaminduced gas-assisted etching or deposition. In many cases, fiducialformation by deposition will be preferable because it is less invasive(causes less damage to the sample surface) and provides a bettercontrast (because a different material is used). The fiducial can bemade of a shape, such as a box or “X,” that is readily distinguishableso that it can be consistently identified and located, especially in alower resolution imaging scan.

In some circumstances, it may be difficult to estimate the desiredlocation for the fiducial marks with sufficient accuracy, and thus itmay be necessary to create the fiducial marks after the proper “row”and/or desired cell location has been determined. In that event, thesample should be re-imaged at a high-resolution to account for sampledrift before the offset between fiducial and desired row or celllocation can be determined.

In step 158, the system (either automatically or manually) can positionthe sample stage so that the center of the field of view is at a middlelocation between the closest corner and the desired cell location. Itmay also be desirable in some circumstances to create the fiducialsdiscussed above after this step. But in that case, the field of viewshould be reimaged at least at the lower resolution (for example, at1K×1K) to account for any error in stage movement.

In step 160, starting at the closest corner, a high-resolution scan isused to acquire an image along the edge of the array, in either the X orY direction. Referring also to FIG. 14, the high-resolution scan couldbe made along the area defined by box 1404 in the Y direction.Preferably, the scan resolution will be at least 16K×1K, which willallow pattern recognition of individual 50 nm cells because pixel sizewill be down to 6 nm. Thus, in step 161, pattern recognition softwarecan be used to count the desired number of cells, for example 399 cellsin the Y direction using the example above. In some preferredembodiments, the cells can be counted as the strip is imaged. In otherpreferred embodiments, a strip will be imaged to a length that willdefinitely include the desired row, and the pattern recognition softwarecan be used to count the desired number of cells in the image.

Once the proper row is identified, the offset between the fiducial andthe row can be optionally recorded in step 162 so that the row can bemore easily relocated when the next scan is acquired. In a system with arelatively large amount of sample drift, a row-marking fiducial isdesirable because it allows accurate placement of the next imaging scan(in optional step 163). In a system without much sample drift, arow-marking fiducial might not be needed. Preferably, the row-markingfiducial, if required, will be placed is a location that will be scannedby both image strips.

Next, in step 164, a high-resolution scan is used to acquire an imagealong the row located in the previous step (for example, in the Xdirection in the example above). Referring also to FIG. 14, thehigh-resolution scan could be made along the area defined by box 1406 inthe X direction. Again, this scan should be at a resolution high enoughto allow pattern recognition on the cells of the array. As above, scanresolution of at least 16K×1K will allow pattern recognition ofindividual 50 nm cells, thus allowing pattern recognition software tocount the desired number of cells as the strip is imaged in step 166.For example, using the XY coordinates above, 2478 cells in the Xdirection could be counted by the pattern recognition software to locatethe desired cell x=2478, y=399 (shown within dashed circle 1407). Forclarity, all of the cells in this example are not shown in FIGS. 14-15.Again, in some preferred embodiments, the cells can be counted as thestrip is imaged. In other preferred embodiments, the strip will beimaged to a length that will definitely include the desired celllocation, and the pattern recognition software can be used to count thedesired number of cells in the image.

To compensate for possible drift during the previous steps, it may bedesirable to repeat the scanning and cell location steps describedabove. Accordingly, in step 168, if necessary to correct for possibledrift, steps 160 to 166 can be repeated.

In step 170, once the location of the desired cell has been determined,the fiducial near the desired cell is identified and the offset betweenthe fiducial and the desired cell is determined and recorded. (In somepreferred embodiments, a plurality of fiducials near the desired cellcan be used.) Whenever an offset between a desired location on this typeof memory array and a fiducial is determined, the offset may beexpressed in numbers of cells (for example, 2 cells down and 1 cell tothe right). In other preferred embodiments, the offset may be expressedin terms of absolute distance or even as relative distance whenfiducials on either side of the target are used.

Once the relationship between the position of the desired cell and thefiducial(s) is known, the desired cell can be easily re-located.Typically, for example, an image with a field of view that is as much as100 μm wide would not be used for particle beam processing of a feature.Instead, a more magnified image having a smaller field of view of, forexample, 10 μm would be used. Accordingly, in step 172, the sample couldbe re-imaged with a smaller field of view (including both the desiredcell and the fiducial) and the recorded offset used to easily locate thedesired cell in the new image (step 174). Thus, even if the stage ismoved or the focus and/or field of view changed, the desired cell can beeasily and rapidly re-located without counting the cells, even when thesample is imaged at a resolution in which the individual cells cannot beidentified (as long as the fiducial can be identified in the image).

It should be noted that in some cases, the process described above mightbe repeated multiple times with the images “stitched” together to findthe actual desired cell. For example, in a very large array, the desiredcell might have an address of y=399, x=7500. The method described abovecould be used to first locate cell address y=399, x=2500 and the offsetbetween than cell and a fiducial recorded. Then a second iteration ofthe method could be used to image a different portion of the array tolocate cell y=399, x=5000, and a third to locate the actual desired celly=399, x=7500.

In some preferred embodiments, the process can be further simplified bylocating the desired cell in one continuous scan, even though only asmall portion of the overall array is actually scanned. For example, apreferred embodiment of the present invention might use a diagonalimage, formed after calculating the cell location along a diagonal linefrom the closest corner (using the XY cell coordinates). This would onlyrequire imaging one strip along the diagonal, using pattern recognitionto perform the cell counting within this diagonal strip, and forming afiducial mark at or near the desired cell. As above, the diagonalimaging could be repeated twice (or more if necessary) to verify thecell location and compensate for sample drift. In another preferredembodiment, the scan along the edge of the array could be scanned, forexample, using an image that is only a few cells wide for some distancealong the edge, and then when the desired row (in which the feature ofinterest is located) is approached, the scan width could be increased toseveral hundred cells, depending upon the X, Y address of the desiredcell.

Preferred embodiments of the present invention provide a significanttimesaving over prior art methods. As discussed above, locating aparticular cell in such a large memory array without using an expensivelaser stage is a time consuming process. Manual cell counting can take10 minutes or more and is prone to errors in counting, which of coursecan mean that the wrong cell is identified. Attempting to automate theprocess by using high-resolution imaging of the entire array (or evenjust the portion of the array containing the feature of interest) wouldbe expected to be even more time consuming. For example, ahigh-resolution scan of an entire 100 μm×100 μm array might take morethan 30 minutes. Using the method of the present invention, however, thetime required for a high-resolution image of only two “strips” of cellsin the array (or four separate strips when the process is repeated tocompensate for drift) would only require around 2 minutes (˜30 sec. perstrip). Adding one minute for forming the fiducial(s) via GIS depositswill bring the total time to 3-4 minutes. Obviously the time could bereduced even further if only one strip (whether diagonal or L-shaped) isimaged as described above.

Preferred embodiments of the present invention do not rely on stageaccuracy performance as would methods using expensive laser stages orstages capable of smooth jogging. This is significant because laserstages or smooth jogging might be made to work now, but would likely notbe accurate enough as nodes and tolerances continue to shrink. Incontrast, embodiments of the present invention are scalable to smallernodes. As performance requirements shrink, it is typically necessary toincrease imaging resolution correspondingly. Embodiments of the presentinvention will still be applicable as long as the achievable imageresolution is great enough to allow pattern recognition of theindividual cells. Although embodiments of the present invention could beused with any type of imaging technology, there are a number oftechniques currently in development or use that will allow SEMresolution to be significantly improved (e.g., UHR mode, fieldstitching, beam deceleration, etc.).

FIG. 12 shows a typical dual beam FIB/SEM system 210 that could be usedto implement preferred embodiments of the present invention. Oneembodiment of the present invention utilizes a dual beam FIB/SEM system210 that uses an ion beam that is either normal or tilted by a fewdegrees to the plane of the sample surface and an electron beam havingan axis that is also tilted, e.g., 52 degrees from the axis of ion beam.In some embodiments, the ion beam and electron beam are capable ofaligning so that the fields of view of both beams are coincident towithin a few microns or less. The ion beam is typically used to imageand machine the work piece, and the electron beam is used primarily forimaging but can also be used for some modification of the work piece.The electron beam will typically produce an image of a higher resolutionthan the ion beam image, and it will not damage the viewed surface likethe ion beam. The image formed by the two beams can look different, andthe two beams can therefore provide more information than a single beam.

Such a dual beam system could be made from discrete components oralternatively, could be derived from a conventional device such as aHelios NanoLab™ system available from FEI Company of Hillsboro, Oreg.The present invention could also be implemented using other particlebeam systems, including for example, single beam systems, such as FIB orSEM only systems, or dual beam systems having two FIB columns.

Focused ion beam system 210 includes an evacuated envelope 211 having anupper neck portion 212 within which are located an ion source 214 and afocusing column 216 including extractor electrodes and an electrostaticoptical system. Ion beam 218 passes from ion source 214 through column216 and between electrostatic deflection means schematically indicatedat 220 toward sample 222, which comprises, for example, a semiconductordevice positioned on movable X-Y-Z stage 224 within lower chamber 226.An ion pump or other pumping system (not shown) can be employed toevacuate neck portion 212. The chamber 226 is evacuated withturbomolecular and mechanical pumping system 230 under the control ofvacuum controller 232. The vacuum system provides within chamber 226 avacuum of between approximately 1×10-7 Torr and 5×10-4 Torr. If an etchassisting, an etch retarding gas, or a deposition precursor gas is used,the chamber background pressure may rise, typically to about 1×10-5Torr.

High voltage power supply 234 is connected to ion source 214 as well asto appropriate electrodes in focusing column 216 for forming an ion beam218 and directing the same downwardly. Deflection controller andamplifier 236, operated in accordance with a prescribed pattern providedby pattern generator 238, is coupled to deflection plates 220 wherebybeam 218 may be controlled to trace out a corresponding pattern on theupper surface of sample 222. In some systems the deflection plates areplaced before the final lens, as is well known in the art.

The ion source 214 typically provides a metal ion beam of gallium,although other ion sources, such as a multicusp or other plasma ionsource, can be used. The ion source 214 typically is capable of beingfocused into a sub one-tenth micron wide beam at sample 222 for eithermodifying the sample 222 by ion milling, enhanced etch, materialdeposition, or for the purpose of imaging the sample 222. A chargedparticle multiplier 240 used for detecting secondary ion or electronemission for imaging is connected to signal processor 242, where thesignal from charged particle multiplier 240 are amplified, convertedinto digital signals, and subjected to signal processing. The resultingdigital signal is to display an image of sample 222 on the monitor 244.

A scanning electron microscope 241, along with power supply and controlunit 245, is also provided with the FIB/SEM system 210. An electron beam243 is emitted from a cathode 252 by applying voltage between cathode252 and an anode 254. Electron beam 243 is focused to a fine spot bymeans of a condensing lens 256 and an objective lens 258. Electron beam243 is scanned two-dimensionally on the specimen by means of adeflection coil 260. Operation of condensing lens 256, objective lens258, and deflection coil 260 is controlled by power supply and controlunit 245.

Electron beam 243 can be focused onto sample 222, which is on movableX-Y-Z stage 224 within lower chamber 226. Scanning electron microscope241 produces a finely focused electron beam 243, which is scanned acrossthe surface of the structure, preferably in a raster pattern. When theelectrons in the electron beam 243 strike the surface of work piece 222,secondary electrons and backscattered electrons are emitted.Respectively, these electrons are detected by secondary electrondetector 240 or backscattered electron detector 262. The analog signalproduced either by secondary electron detector 240 or backscatteredelectron detector 262 is amplified and converted into a digitalbrightness value by signal processor unit 242. The resulting digitalsignal can be displayed as an image of sample 222 on the monitor 244.

A door 270 is opened for inserting sample 222 onto stage 224, which maybe heated or cooled, and also for servicing an internal gas supplyreservoir, if one is used. The door is interlocked so that it cannot beopened if the system is under vacuum. The high voltage power supplyprovides an appropriate acceleration voltage to electrodes in ion beamcolumn 216 for energizing and focusing ion beam 218.

A gas delivery system 246 extends into lower chamber 226 for introducingand directing a gaseous vapor toward sample 222. U.S. Pat. No. 5,851,413to Casella et al. for “Gas Delivery Systems for Particle BeamProcessing,” assigned to the assignee of the present invention,describes a suitable gas delivery system 246. Another gas deliverysystem is described in U.S. Pat. No. 5,435,850 to Rasmussen for a “GasInjection System,” also assigned to the assignee of the presentinvention. For example, iodine can be delivered to enhance etching, or ametal organic compound can be delivered to deposit a metal.

System controller 219 controls the operations of the various parts ofdual beam system 20. Through system controller 119, a user can cause ionbeam 218 or electron beam 143 to be scanned in a desired manner throughcommands entered into a conventional user interface (not shown). Systemcontroller 119 can also comprise computer-readable memory 221 and maycontrol dual beam system 110 in accordance with data or programmedinstructions stored in memory 221. CAD data concerning thesample/semiconductor stored in memory 221 can be used to create a CADpolygon overlay or other positional data used to locate a feature ofinterest and alignment points or transfer fiducials as described above.

Although the description of the present invention above is mainlydirected at a method of high-accuracy beam placement for local areanavigation, it should be recognized that an apparatus performing theoperation of this method would further be within the scope of thepresent invention. Further, it should be recognized that embodiments ofthe present invention can be implemented via computer hardware orsoftware, or a combination of both. The methods can be implemented incomputer programs using standard programming techniques—including acomputer-readable storage medium configured with a computer program,where the storage medium so configured causes a computer to operate in aspecific and predefined manner—according to the methods and figuresdescribed in this Specification. Each program may be implemented in ahigh level procedural or object oriented programming language tocommunicate with a computer system. However, the programs can beimplemented in assembly or machine language, if desired. In any case,the language can be a compiled or interpreted language. Moreover, theprogram can run on dedicated integrated circuits programmed for thatpurpose.

Further, methodologies may be implemented in any type of computingplatform, including but not limited to, personal computers,mini-computers, main-frames, workstations, networked or distributedcomputing environments, computer platforms separate, integral to, or incommunication with charged particle tools or other imaging devices, andthe like. Aspects of the present invention may be implemented in machinereadable code stored on a storage medium or device, whether removable orintegral to the computing platform, such as a hard disc, optical readand/or write storage mediums, RAM, ROM, and the like, so that it isreadable by a programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. Moreover, machine-readablecode, or portions thereof, may be transmitted over a wired or wirelessnetwork. The invention described herein includes these and other varioustypes of computer-readable storage media when such media containinstructions or programs for implementing the steps described above inconjunction with a microprocessor or other data processor. The inventionalso includes the computer itself when programmed according to themethods and techniques described herein.

Computer programs can be applied to input data to perform the functionsdescribed herein and thereby transform the input data to generate outputdata. The output information is applied to one or more output devicessuch as a display monitor. In preferred embodiments of the presentinvention, the transformed data represents physical and tangibleobjects, including producing a particular visual depiction of thephysical and tangible objects on a display.

Preferred embodiments of the present invention also make use of aparticle beam apparatus, such as a FIB or SEM, in order to image asample using a beam of particles. Such particles used to image a sampleinherently interact with the sample resulting in some degree of physicaltransformation. Further, throughout the present specification,discussions utilizing terms such as “calculating,” “determining,”“measuring,” “generating,” “detecting,” “forming,” or the like, alsorefer to the action and processes of a computer system, or similarelectronic device, that manipulates and transforms data represented asphysical quantities within the computer system into other data similarlyrepresented as physical quantities within the computer system or otherinformation storage, transmission or display devices.

The invention has broad applicability and can provide many benefits asdescribed and shown in the examples above. The embodiments will varygreatly depending upon the specific application, and not everyembodiment will provide all of the benefits and meet all of theobjectives that are achievable by the invention. Particle beam systemssuitable for carrying out the present invention are commerciallyavailable, for example, from FEI Company, the assignee of the presentapplication. However, even though much of the previous description isdirected toward the use of FIB milling and imaging, the beam used toprocess the desired samples could comprise, for example, an electronbeam, a laser beam, or a focused or shaped ion beam, for example, from aliquid metal ion source or a plasma ion source, or any other chargedparticle beam. Further, although much of the previous description isdirected at particle beam systems, the invention could be applied to anysuitable sample imaging system employing a moveable sample stage tonavigate to the location of a sample feature.

Although much of the previous description is directed at semiconductorwafers, the invention could be applied to any suitable substrate orsurface. Further, whenever the terms “automatic,” “automated,” orsimilar terms are used herein, those terms will be understood to includemanual initiation of the automatic or automated process or step.Whenever a scan or image is being processed automatically using computerprocessing, it should be understood that the raw image data can beprocessed without ever generating an actual viewable image. The term“image” is used in a broad sense to include not only a displayed imageshowing the appearance of the surface, but also to include anycollection of information characterizing the multiple points on or belowa surface. For example, a collection of data corresponding to thesecondary electrons collected when a particle beam is at different pointon a surface is a type of “image,” even if the data is not displayed.Collecting information about points on the sample or workpiece is“imaging.”

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .” The term“integrated circuit” refers to a set of electronic components and theirinterconnections (internal electrical circuit elements, collectively)that are patterned on the surface of a microchip. The term“semiconductor device” refers generically to an integrated circuit (IC),which may be integral to a semiconductor wafer, singulated from a wafer,or packaged for use on a circuit board. The term “FIB” or “focused ionbeam” is used herein to refer to any collimated ion beam, including abeam focused by ion optics and shaped ion beams.

When the positional error or accuracy of the system stage or of beamplacement or navigation is discussed herein, the terms ±100 nm (or ±30nm or ±X nm) mean that the beam can be directed at a location on thesample within a maximum error of 100 nm (or 30 nm or x nm). The terms“accuracy of ±X nm” or “positioning accuracy of X nm or better” meansthat the accuracy is at least X nm and includes all smaller values. Theterm “accuracy of X nm or greater” means that the accuracy is at best Xnm and includes all larger values.

To the extent that any term is not specially defined in thisspecification, the intent is that the term is to be given its plain andordinary meaning. The accompanying drawings are intended to aid inunderstanding the present invention and, unless otherwise indicated, arenot drawn to scale.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods, and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

We claim as follows:
 1. A method for high accuracy beam placement andnavigation to a feature of interest having a known cell address X, Ywithin an array of cells on a sample surface, comprising: loading asample into a particle beam system having one or more particle beams;acquiring, using at least one of the particle beams, an image of atleast a portion of the array at a first image resolution, the imagehaving a field of view large enough to include the location of thefeature of interest and at least one corner of the array; forming, usingat least one of the particle beams, at least one fiducial on the samplesurface at a location near the estimated location of the feature ofinterest; obtaining, using at least one of the particle beams, an imageof an edge strip along an edge of the array, said edge strip imagehaving a resolution high enough that the pixel size is less than half ofthe minimum repeating dimension of the cells in the array, and the edgestrip image being at least X cells in length and substantially smallerthan the field of view in the first image; using pattern recognition toautomatically count X cells to determine the location of the desired rowcontaining the feature of interest; obtaining, using at least one of theparticle beams, a row strip image along the desired row, said row stripimage having a resolution high enough that the pixel size is less thanhalf of the minimum repeating dimension of the cells in the array, andthe row strip image being at least Y cells in length and substantiallysmaller than the field of view in the first image, and the row stripimage including at least one fiducial and the feature of interest; usingpattern recognition to automatically count Y cells along the desired rowto determine the location of cell address X, Y containing the feature ofinterest; and determining the offset between the at least one fiducialand the location of cell address X, Y containing the feature ofinterest.
 2. The method of claim 1 in which said first image resolutionis low enough that the image pixel size is larger than the maximumfeature dimension of the cells in the array.
 3. The method of claim 1 inwhich acquiring a first image of at least a portion of the arraycomprises acquiring a first image of an area at least 100 μm×100 μm. 4.The method of claim 1 in which said first image resolution is 1K×1Kpixels or lower.
 5. The method of claim 1 in which the maximum featuredimension of the cells in the array is 50 nm or less and the first imageresolution has a pixel size of 200 nm or more.
 6. The method of claim 1in which said second image resolution is high enough that the pixel sizeis less than half of the maximum feature dimension of the cells in thearray.
 7. The method of claim 1 in which the maximum feature dimensionof the cells in the array is 10 to 50 nm and the second image resolutionhas pixel size of less than 5 nm.
 8. The method of claim 1 in which saidsecond image resolution is 16K×1K pixels or higher.
 9. The method ofclaim 1 in which forming at least one fiducial on the sample surfacecomprises forming at least one fiducial on the sample surface having asize and shape that is readily distinguishable at the first imageresolution.
 10. The method of claim 1 in which the at least one fiducialis readily distinguishable at a resolution of 1K×1K pixels.
 11. A methodfor high accuracy beam placement and navigation to a feature of interesthaving a known cell address X, Y within an array of cells on a samplesurface, comprising: loading a sample into a particle beam system havingone or more particle beams; navigating at least one of the particlebeams to the portion of the array containing the feature of interest;forming, using at least one of the particle beams, at least one fiducialon the sample surface at a location near the estimated location of thefeature of interest; obtaining, using at least one of the particlebeams, an image of an edge strip along an edge of the array, said edgestrip image having a resolution high enough that the pixel size is lessthan half of the minimum repeating dimension of the cells in the array,and the edge strip image being at least X cells in length andsubstantially smaller than the field of view in the first image;analyzing the image data to automatically count X cells along the edgeof the array to determine the location of the desired row containing thefeature of interest; obtaining, using at least one of the particlebeams, a row strip image along the desired row, said row strip imagehaving a resolution high enough that the pixel size is less than half ofthe minimum repeating dimension of the cells in the array, and the rowstrip image being at least Y cells in length and substantially smallerthan the field of view in the first image, and the row strip imageincluding at least one fiducial and the feature of interest; analyzingthe image data to automatically count Y cells along the desired row todetermine the location of cell address X, Y containing the feature ofinterest; and determining the offset between the at least one fiducialand the location of cell address X, Y containing the feature ofinterest.
 12. The method of claim 11 in which forming at least onefiducial on the sample surface comprises forming at least one fiducialon the sample surface by at least one of focused ion beam sputtering,surface staining with an ion beam, charged particle beam inducedgas-assisted etching, or charged particle beam induced gas-assisteddeposition.
 13. The method of claim 11 in which forming at least onefiducial on the sample surface at a location near but separated from theestimated location of the feature of interest comprises forming a firstfiducial near but separated from said desired row and a second fiducialnear but separated from said estimated location of the feature ofinterest.
 14. The method of claim 11 further comprising, after the stepof automatically counting X cells to determine the location of thedesired row containing the feature of interest, determining the offsetbetween the first fiducial and the desired row.
 15. The method of claim11 further comprising, after determining the offset between a fiducialand the location of cell address X, Y containing the feature of interestre-imaging the sample using a smaller field of view, said smaller fieldof view including the fiducial and the location of cell address X, Y,and using the previously determined offset between the fiducial and celladdress X, Y to locate cell address X, Yin the image having a smallerfield of view.
 16. The method of claim 11 further comprising, afterdetermining the location of cell address X, Y, and before the step ofdetermining the offset in the fourth image between the fiducial and thelocation of cell address X, Y, compensating for system drift byrepeating the steps of obtaining an image of an edge strip,automatically counting X cells along the edge of the array to determinethe location of the desired row containing the feature of interest,obtaining a row strip image along the desired row, and automaticallycounting Y cells along the desired row to determine the location of celladdress X, Y containing the feature of interest.
 17. The method of claim11 further comprising, after determining the offset in the fourth imagebetween the fiducial and the location of cell address X, Y, moving thestage or changing the focus or field of view; re-imaging the at least aportion of the array; identifying the at least one fiducial in the imageof the at least a portion of the array; and determining the location ofcell address X, Y from the determined offset.
 18. The method of claim 11in which the array of cells on a sample surface comprises a memory arrayon a semiconductor wafer.
 19. The method of claim 11 in which forming atleast one fiducial on the sample surface at a location near theestimated location of the feature of interest comprises forming at leastone fiducial on the sample surface at a location near but separated fromthe estimated location of the feature of interest.
 20. The method ofclaim 11 in which forming at least one fiducial on the sample surfacecomprises forming at least one fiducial on the sample surface before thesample is loaded into the particle beam system.
 21. A method for highaccuracy beam placement and navigation to a feature of interest in acell having a known cell address within an array of cells on a samplesurface, comprising: loading a sample into a particle beam system havingone or more particle beams; navigating at least one of the particlebeams to the portion of the array containing the feature of interest;forming, using at least one of the particle beams, at least one fiducialon the sample surface at a location near the estimated location of thefeature of interest; obtaining, using at least one of the particlebeams, an image of a strip of cells in the array, said strip imagehaving a resolution high enough that the pixel size is less than half ofthe minimum repeating dimension of the cells in the array, and the stripimage including the feature of interest, the at least one fiducial, andat least one corner of the array; analyzing the image data toautomatically count cells in the strip image to determine the locationof the cell containing the feature of interest; and determining theoffset between the at least one fiducial and the location of the cellcontaining the feature of interest.
 22. The method of claim 21 in whichobtaining an image of a strip of cells in the array comprises obtainingan image of a strip of cells in the array along a diagonal line betweena corner of the array and the location of the cell containing thefeature of interest.
 23. The method of claim 21 in which obtaining animage of a strip of cells in the array comprises obtaining an image ofan L-shaped strip of cells in the array.
 24. The method of claim 23 inwhich the location of the cell containing the feature of interest iscell address X, Y, and in which obtaining an image of an L-shaped stripof cells in the array comprises obtaining an image of an L-shaped stripof cells in the array, said strip being only a few cells wide from thecorner of the array until it approaches the row X, but being at least Ycells wide at row X.